Abstract: The relation between cut and optical effect of diamond is so complicated that computer 3D emulation model is necessary when it is lucubrated. The basic principle and constituent parts of the diamond 3D emulation system are introduced in the paper. The basic principle is light tracing. The system consists of four parts: light source model, diamond model, observation model and light tracing module. In addition, two numeral indexes are defined: brightness return(BR) and coloured light return(CR), which evaluate the target of good brilliance and good fire on diamond products.